As a regulatory thing,
US FAR 25.1553 states:
If the unusable fuel supply for any tank exceeds one gallon, or five percent of the tank capacity, whichever is greater, a red arc must be marked on its indicator extending from the calibrated zero reading to the lowest reading obtainable in level flight
Given that the 737 fuel guages don't have a red arc, they either don't show unuseable fuel, or that fuel is less than a gallon...